The present invention relates to a wheel alignment measuring apparatus.
In a paper in "Krafthand" no. 18, 1978, with the title "Achsvermessung-grundlegende Neuerung" (a completely new look at measuring wheel alignment), it was said that the development of a new way of measuring wheel alignment has become necessary for overcoming the shortcomings, detailed in the paper, of optical systems used so far in the art, and more importantly, the lining up of the vehicle in an optical four-cornered figure. In the paper, the writer said that for meeting present day needs such wheel alignment apparatus is now to be designed for measuring not only camber, caster, toe-in and toe-out (as is usual practice) but also the single-wheel tracking angles of the front wheels in relation to the geometrical driving axis of the vehicle, the single-wheel tracking angles of the rear wheels with respect to the axis of symmetry of the vehicle, the total tracking angle of the front wheels and the total tracking angle of the rear wheels. The axis of symmetry is defined as an imaginary line, stretching from the center of the front axle to the center of the rear axle. The zero value for the rear wheel single-wheel tracking angle is the position in which the wheel axis is normal to the axis of symmetry. The geometrical driving axis is defined as the line bisecting the rear wheel total tracking angle or, in other words, an imaginary line relative to which the rear wheels have equal inclination. The zero value of the single-wheel tracking angle for the front wheels is the position in which the wheel axis is normal to the geometrical driving axis. On the other hand, the difference in track, that is to say the difference between the tracks of the front and rear wheels, and differences in wheel base, that is to say from the front to the rear axle are not to be taken into account in measuring operations, that is to say they are not to have any effect on the readings produced.
In the prior art, a group of forms of measuring apparatus has been produced with which one or more wheel alignment angles may be measured, see the French Pat. No. 2,343,221, the U.S. Pat. No. 4,126,943, the British Pat. No. 1,372,064, the German Pat. No. 2,935,899, the German Pat. No. 2,926,337, the Swiss Pat. No. 424,288 and the U.S. Pat. No. 3,417,479. These prior forms of measuring apparatus are generally overcomplex and may not be used for measuring the wheel alignment angles needed. In the U.S. Pat. No. 4,126,943 and the French Pat. No. 2,343,221, it is only the front wheels which have two measuring units (projectors) placed on them, the rear wheels only having reflectors for the projectors on the front wheels. For this reason, the angle alignment of the rear wheels is not fully taken into account in the measuring procedure so that also not all of the requirements can be met which are imposed on present-day wheel alignment measuring systems. In the apparatus of the British Pat. No. 1,372,064 and of German Pat. No. 2,926,337, only the alignment of the front wheels is measured, the position and angle of the rear wheels not being taken into account. In the German Pat. No. 2,926,337, as well, the position and angle of the rear wheel is not fully taken into account, although two measuring units, placed on the front wheels, are joined up by way of flexible cables with parts gripping the rear wheels. For this reason, the three last-named apparatus of the prior art may not be said to be in line with the conditions supra, which have to be kept to. Lastly, the Swiss Pat. No. 424,288 and the U.S. Pat. No. 3,417,479 are with respect to apparatus making a rigid frame necessary, if the alignment or angles of all four wheels are to be measured. Such a measuring frame may hardly, because of its size, be moved from place to place and may, in fact, only be used in one given position and not at a number of different positions in a vehicle repair station.